Television and like camera tubes



Nov. 12, 1963 E. B. B. cALLlcK ETAL 3,110,841

TELEVISION AND LIKE CAMERA TUBES Filed Jan. 25. 1.960

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A'r-roRN EYs United States Patent O 3,110,841 n TELEVESEON AND LiKE CAMERA TUBES Erle Brian Butler Calliclr, Cheimsford, and Jeri/Jois Camphell Firmin, Little Batidora', England, assignors to Englush Eiectric Valve Company Limited, Loudon, England, a British company Filed Jan. 25, 196i), Ser. No. 4,285 Claims priority, appiication Great Eritain Feb. 2, 1959 Claims. (Cl. 315-10) This invention relates to television and like camera tubes, that is to say to tubes for translating optical images of subjects of transmission into electrical video signals. The object of the invention is to provide television and like camera tubes which shall be of relatively simple and inexpensive construction and good sensitivity and which shall not impose requirements of extreme precision in manufacture. A further object of the invention is to provide improved camera tubes which will be almost if not quite free of so-called dark current effect and which will in fact give maximum output current for white parts of a subject for transmission and very low (approaching zero) output current for black parts thereof.

According to this invention a television or like camera tube includes a mesh electrode with a photo-sensitive layer on one side thereof positioned to receive optical images of a subject of transmission, an electron gun adapted to scan said side of said mesh electrode with an electron beam, an additional electrode positioned to receive beam electrons which have passed through said mesh electrode and means including a second additional electrode for receiving beam electrons which have been turned back by said mesh electrode.

The electrode positioned to receive beam electrons which have passed through the mesh may be the actual output electrode of the tube but preferably it is an accelerating grid for an electron multiplier system of dynodes leading to an output electrode and adapted to supply an increased electron flow thereto.

The photo-sensitive layer may be a photo-emissive insulating layer or a photo-conductive layer.

When a tube in accordance with this invention is operating, the operating potentials on the various electrodes are made such that the number of 'beam electrons passed by said mesh electrode at any particular small area of its surface depends on the light content of the portion of the optical image on said small area.

In a preferred construction the means including the second additional electrode for receiving beam electrons turned back by the mesh electrode is a grid or mesh (to which, in use, a positive potential is applied) which is adjacent said mesh electrode on the electron gun side thereof and through which the beam passes to reach said mesh electrode and preferably a first additional electrode in the form of a grid or mesh (to which, in use, a higher positive potential is applied) is provided adjacent said mesh electrode on the side thereof remote from the electron gun and between said mesh electrode and the electrode for receiving beam electrons which have passed through said mesh electrode.

The invention is illustrated in the accompanying drawing which is a diagrammatic representation of one embodiment thereof. In the drawing, certain practical applied potentials are indicated, but these are by way of example only.

Referring to the drawing, the camera tube therein represented has an evacuated envelope 1 with a window 2 at one end. Transverse with respect to the envelope is a fine metal mesh or apertured screen 3 which is represented by a broken line and on one side of which is a photo-sensitive layer 4 represented by a second broken line. The layer 4 may be either a photo-emissive insulating layer or a photo-conductive layer. Optical images of a subject for transmission (not shown) are focused by any convenient means represented by the lens 5 through a window 2 at one end of the tube envelope on to the photosensitive layer 4 0n the mesh 3. Adjacent the structure 3 4 and on the window side thereof'is an additional electrode in the form of a grid or mesh 6 and also adjacent the structure 3 4 and on the opposite side thereof is another additonal electrode in the form of a grid or mesh '7. An electron gun 8 is provided to produce an electron beam to scan the photo-sensitive surface 4 on the mesh 3. Behind the mesh 3 and on the other side of the grid 7 therefrom is an electron multiplier system represented conventionally by a sequence of electron multiplying electrodes or dynodes 9 terminating in an output electrode 10. Any convenient scanning arrangement indicated by scanning coil systems 11 and 12 in the ligure is provided for causing the beam from the gun 8 passing through the grid 6 to scan the structure 3-4, electrons of this beam which have passed through said structure and through the grid 7 being caused to be incident upon the list dynode in the sequence of dynodes.

In use suitable positive potentials, for example as indicated in the figure, are applied to the grids 6 and '7 from D.C. source 17 and before any optical image is projected on to the layer 4 a positive voltage of, for example, l0 volts is applied to the mesh 3. The electron beam from the gun 8 is then caused to scan the surface 4 thus reducing that surface to zero (i.e. gun cathode) potential. The mesh 3 is then connected to earth which brings the photo-sensitive surface 4 to a negative potential of about -10 volts (again relative to gun cathode). This is the setting-up process which brings the tube into condition for use and the means for effecting it are conventionally indicated by the switch 13, though it will be understood, of course, that in practice, setting-up will probably be effected by a suitable pulsed waveform.

With the tube thus set-up, optical images of a subject for transmission are projected on to the layer 4 and the said layer is scanned by the beam from the gun. It will be seen that electrons of the beam will pass through the mesh 3 only at those points where the local surface potential has been brought above the beam cut-off value as a result of emission of photo electrons (if the surface 4 is photo-emissive) or of photo-conductivity (if the surface 4 is a photo-conductive layer) and this will occur at the white areas of the image. At black areas of the image the local surface potential on the mesh will be below cut-off value and beam electrons will not pass through the mesh but be returned to the grid 6. The purpose of this grid is to collect returned electrons While the purpose of the grid 7 is to draw electrons through the mesh at points where the mesh potential is above cut-off value. Thus the electron ow through the mesh at any point will depend on the light or shade of the image at that point. Electrons passing through the mesh and the grid 7 impinge on the dynode succession each electrode 9 of which produces an electron mutiplication effect, the final amplified electron stream falling upon an output electrode 10. Picture signals are taken olf through condenser 14 from across an output resistance 15. The electron beam is conventionally represented by the chain line 16.

The tube above described is relatively simple, of good sensitivity and does not impose excessively stringent requirements as to precision of manufacture-for example, the optical system represented by the external lens 5 need not be exactly co-axial with the gun 8. Furthermore, it will be seen that little or no beam current will reach the electron multiplier section of the tube at points on the structure 3 4 corresponding to dark or black areas of the picture so that there is little or no dark current effect.

The electron multiplier included in the tube illustrated is not a theoretical necessity and the electron beam passing through the mesh 3 could be arranged to be incident directly on an output electrode, for example, a plate electrode parallel to and behind the grid 7, i.e. on the side of the grid 7 remote from the gun. However, the illustrated arrangement is preferred.

We claim:

1. A television camera tube arrangement comprising a tube having a mesh electrode with a photosensitive layer on one side thereof positioned to receive optical images, an electron gun including a cathode adapted to project an electron beam in a direction perpendicular to the surface of said mesh electrode to scan said layer, an electrode positioned to receive beam electrons which have passed through said mesh electrode, a pair of grid electrode means positioned adjacent and parallel to said mesh electrode, one on each side thereof, means for stabilizing the potential of said photosensitive layer at a predetermined value, a source of D.C. potential, and means for connecting each of said grid electrode means and said cathode to said source to render said grid electrode means positive with respect to said cathode, whereby said grid electrode means positioned on Ithe side of the mesh elcctrode nearer the electron gun will receive electrons emitted by or turned back by the mesh electrode and the other grid electrode means will draw electrons through said mesh electrode at points where the mesh potential is above the cut-ott value.

2. A tube as ciaimed in claim 1 wherein the photosensitive layer is a photo-emissive insulating layer.

3. A tube as claimed in claim 1 wherein the photosensitive layer is a photo-conductive layer.

4. A tube as claimed in claim 1 wherein the electrode positioned to receive beam electrons which have passed through the mesh is the actual output electrode of the tube.

5. A tube as claimed in claim 1 wherein the electrode positioned to receive beam electrons which have passed through the mesh is the first electrode of an electron multiplier system of dynodes leading to an output electrode.

References Cited in the tile of this patent UNITED STATES PATENTS 2,256,300 Van Mierlo Sept. 16, 1941 2,618,758 Cage Nov. 18, 1952 2,816,954 Huffman Dec. 17, 1957 

1. A TELEVISION CAMERA TUBE ARRANGEMENT COMPRISING A TUBE HAVING A MESH ELECTRODE WITH A PHOTOSENSITIVE LAYER ON ONE SIDE THEREOF POSITIONED TO RECEIVE OPTICAL IMAGES, AN ELECTRON GUN INCLUDING A CATHODE ADAPTED TO PROJECT AN ELECTRON BEAM IN A DIRECTION PERPENDICULAR TO THE SURFACE OF SAID MESH ELECTRODE TO SCAN SAID LAYER, AN ELECTRODE POSITIONED TO RECEIVE BEAM ELECTRONS WHICH HAVE PASSED THROUGH SAID MESH ELECTRODE, A PAIR OF GRID ELECTRODE MEANS POSITIONED ADJACENT AND PARALLEL TO SAID MESH ELECTRODE, ONE ON EACH SIDE THEREOF, MEANS FOR STABILIZING THE POTENTIAL OF SAID PHOTOSENSITIVE LAYER AT A PREDETERMINDED VALUE, A SOURCE OF D.C. POTENTIAL, AND MEANS FOR CONNECTING EACH OF SAID GRID ELECTRODE MEANS AND SAID CATHODE TO SAID SOURCE TO RENDER SAID GRID ELECTRODE MEANS POSITIVE WITH RESPECT TO SAID CATHODE, WHEREBY SAID GRID ELECTRODE MEANS POSITIONED ON THE SIDE OF THE MESH ELECTRODE NEARER THE ELECTRON GUN WILL RECEIVE ELECTRONS EMITTED BY OR TURNED BACK BY THE MESH ELECTRODE AND THE OTHER GRID ELECTRODE MEANS WILL DRAW ELECTRONS THROUGH SAID MESH ELECTRODE AT POINTS WHERE THE MESH POTENTIAL IS ABOVE THE CUT-OFF VALUE. 